Conventional local area networks comprising a plurality of computing entities, for example personal computers (PCs) transmit and receive signals with each other according to known protocols, such as the Ethernet protocol, over coaxial cables connected between Ethernet ports provided at each of the individual computing entities. Whilst cabled network solutions are very successful commercially and technically, they have a disadvantage in flexibility of usage and cost. For example in a typical office environment provision needs to be made for cabling between computing entities, such as raised flooring. Although a cable solution itself is relatively inexpensive, there are hidden costs in provided ducting for cabling between computing entities.
A known solution which removes the need for coaxial cable extending between different computing entities in a local area network involves each computing entity being provided with a transmitter/receiver device which operates at wireless frequencies, typically 5 GHz. Such short range communications are practical within enclosed indoor environments for communicating over short distances of the order of meters to tens of meters at relatively low power. A plurality of computing entities linked together in a local area network use wireless links to communicate with each other. Within a particular network, the plurality of entities all communicate with each other on a single frequency channel, of frequency of the order 5 GHz using a CSMA protocol in which a sending entity transmits a plurality of data packets with all entities in a network receiving the data packets at the same carrier frequency. The packet contains a header information which includes an address of a particular computing entity for which the packet is intended. The address information is added by a higher level protocol than CSMA. Only the computing entity whose address is included in the header decodes the packet. According to the CSMA protocol, to avoid two or more computing entities transmitting at the same time on a same frequency, the CSMA protocol includes transmission rules which allows or denies each computing entity permission to transmit. Therefore only one computing entity transmits at any one time, and all computing entities receive the transmission, but only computing entities to whom the packet header is addressed decode the packets.
Referring to FIG. 1 herein, there is illustrated schematically first and second computing entities 100, 101 in an indoor office environment communicating with each other. First computing entity 100 comprising a personal computer transmits a message for printing a document, to second computing entity 101, in this example a printer device. In order for the computing entities to be physically moved around relative to each other so that computers, printers, peripheral devices and the like comprising the computing entities in the network can be placed anywhere within a local area, each entity transmits and receives omni-directionally. However, in an indoor environment, such as an office, domestic premises or laboratory there exist a large number of obstacles and reflective surfaces, including walls, heating radiators, doors, ceilings, floorings, filing cabinets and the like, all of which reflect transmissions. Further, the reflective properties of the indoor environment may change dynamically, for example with people walking in and out of the environment, doors or windows opening and closing, new objects being introduced into the area or existing objects being removed from the area.
Receiving entity 101 may receive transmissions from transmitting entity 100 over a large plurality of transmission paths due to reflections in the environment of the local area network, as illustrated schematically by path arrows 102–105 in FIG. 1. Receiving entity 101 receiving multi-path transmission may experience fading due to out-of-phase cancellation from transmissions received over different paths. The problem of fading increases with the rate of data transmission over the network, since inter-symbol interference between received digital pulses increases with increasing data rate, and is also dependent upon the pulse period and the period between the transmission of individual pulses.